2275 dizes iodide ion to iodine and triphenylphosphine to triphenylphosphine oxide. Scheme I summarizes some other reactions of 4 which have been studied." All reactions were carried out in THF solution. With the alcohols and with water, exothermic reactions a t room temperature were observed. In the case of the reaction with methyllithium, higher oligomers, u p to THF-insoluble-polymer, were formed. These must have resulted from attack of the intermediate lithium reagent, MesSiC(CH3)2C(CH3)2Li, a t the silacyclopropane ring. For all new compounds combustion analyses and spectroscopic data were obtained which were in full agreement with the structures indicated.
Acknowledgments. The authors are grateful to the U S . Air Force Office of Scientific Research (NC)-AFSC (Grant AF-AFOSR 72-2204) for generous support of this work and to Mrs. Mara 0. Nestle for assistance in obtaining carbon- 13 and silicon-29 N M R spectra.
8.
6A6
A6
References and Notes (1) R. L. Lambert, Jr., and D. Seyferth. J. Am. Chem. SOC., 94, 9246
(1972). (2)D. Seyferth, C. K. Haas, and D. C. Annarelli, J. Organomet. Chem., 56, C7 (1973). The X-ray crystal structure of one of the isomers 01 1 has been determined and the endocyclic C-Si-C angle was found to be 49': G. D. Stucky, private communication. References to previously reported attempts to prepare silacyclopropanes are collected in ref 1. Without doubt, other unsuccessful attempts remain unpublished. C. Eaborn, J. Chem. SOC.,2755 (1949). This reaction is based on the known bromination of dimethylisopropylchiorosllane: F. K. Cartledge and J. P. Jones, J. Organornet. Chem., 67,
379 (1974). Determined by quenching aliquots of the hexamethylsilirane-THF sobtion with anhydrous methanol at various times and determining the yield of Me2CHCMe2SiMe20Meformed by GLC. NMR techniques also are applicable. The decomposition products have not yet been isolated. NOTE ADDED IN PROOF. A referee has suggested that to aSSeSS Steric VS. electronic factors 1,1,2.3-tetramethyl-2,3diisopropylsllirane or 1,l-dimethyi-2.2,3,3-tetraethyisiiiranewould be better compounds than hexamethylsiiirane for the comparison with 2. Although hexamethylsilirane is somewhat less hindered than 2, we feel it is a reasonably good first approximation for the purpose under discussion.The two compounds suggested are more hindered than 2 since the alkyl substituents on the carbon atoms are not "tied back" as in the dispiro system. P. D. Mollere and R. Hoffmann. J. Am. Chem. SOC.,submitted. M. lshikawa and M. Kumada, J. Organomet. Chem., 81, C3 (1974). Indicated yields are based on the amount of dibromo precursor used.
Dietmar Seyferth,* Dennis C. Annarelli Department of Chemistry Massachusetts Institute of Technology Cambridge, Massachuserts 021 39 Received January 15, 1975
Nuclear Magnetic Resonance Studies of Metal Complexes Using Lanthanide Shift Reagents. Lanthanide Induced Shifts in the Spectra of Oxygen-Donor Ligands Coordinated to Nickel(I1) Sir.
Since the introduction of lanthanide shift reagents ( L S R ) in 1969,' these reagents have been widely used to simplify the N M R spectra of organic compounds and examples of their use with organometallic compounds have also recently appeared.' W e now report the successful use of these reagents to induce shifts in the spectra of coordinated ligands in coordination complexes containing oxygen donors. T h e effect of added shift reagent on the ' H N M R spectra of a range of diamagnetic nickel complexes of type 1 has been studied. The spectrum of N,N'-ethylenebis(acety1acetonato)nickel(ll) ( I , R = CH3) in deuteriochloroform has been pre-
Figure 1. Comparison of the relative chemical shifts ( S )induced by Eu(fod)3 and Pr(fod)3 in the N M R spectrum of (1, R = CH3).
pR A I
R
1
viously described3 and contains single peaks ( E values) a t 1.87 (-CH3), 3.07 (-CH2-), and 4.89 (=CH-) ppm. On successive additions of Eu(fod)3 or Pr(fod)3 to this compound, the proton resonance shifts illustrated in Figure 1 are observed. As is usually the case with organic compounds, Eu(fod)3 and Pr(fod)3 cause a particular proton resonance to shift in opposite directions. The relative orders of the shifts in Figure 1 are in accord with the LSR interacting with the cis oxygen atoms of the nickel complex as shown by 11; however, the possibility that the interaction is monodentate i n nature and is associated with a rapid equilibrium between oxygen donor sites on the nickel complex cannot be disniissed on the present evidence. Nevertheless, the ability of europium and praseodymium to expand their coordination numbers from six to eight has been well d ~ c u m e n t e d and ,~ the electron lone pairs associated with such cis oxygen donors i n related complexes have been shown to be capable of coordinating simultaneously to a second metal ion.5 Examples of difunctional organic molecules acting as bidentate ligands toward LSR's are also k n ~ w n . ~ ~ ~ It is pertinent that both Eu(fod)3 and Pr(fod)3 have been shown to self-associate (in cc14) via bridging P-diketone oxygen donors in a manner which appears analogous to the interactions reported here.8 Although a general trend of diminution of shift with distance from LSR is apparent (Figure I ) , clearly a detailed analysis of the magnitude of the shifts associated with bidentate systems of the present type is inappropriate in terms of the McConnell-Robertson e q ~ a t i o n . ~ Communications t o the Editor
2276 R
CH2CH(CH&
2
A6
J. E. Schwarberg, D. R. a r e , R. E. Sievers, and K. J. Eisentraut. horg. Chem., 8, 1933, (1969); R. E. Cramer and K. Seff. Acta Crystallogr.. Sect. 6, 28, 3281 (1972); J. H. Forsberg. Coord. Chem. Rev.. 10, 195 (1973). (5) E. Sinn and C. M. Harris, Coord. Chem. Rev., 4, 391 (1969). (6) G. E. Wright, and T. Y. Tang Wei. Tetrahedron,3775 (1973). (7) N. S. Bhacca, J. Selbin. and J. D. Wander, J. Am. Chem. SOC., 94, 8719 (1972). (8) A. H. Bruder. S. R . Tanny. H. A. Rockefeller, and C. S. Springer, Inorg. Chem., 13, 880 (1974). (9) R. Wei, and S. Cummings, Inorg. Nucl. Chem. Led., 9, 43 (1973). (10) B. C. Mayo, Chem. SOC.Rev.. 49 (1973).
L. F. Lindoy,* W. E. Moody Department of Chemistry and Biochemistry James Cook University of North Queensland Post Office, James Cook University Queensland 481 I , Australia Received December 31, 1974
1
Ph
0
1-Bu I
I
Figure 2. Comparison of the relative chemical shifts of the central proton of the 0-h donor chelate rings with change of R for complexes of t)pc 1 on addition of Eu(fod)3.
Further studies involving complexes of type 1 in which R becomes successively more bulky also confirm attack of the LSR at the oxygen donors. As the size of R increases, the attachment of the LSR becomes hindered and the magnitude of the shifts of respective resonances decreases for a given L.SR:complex ratio (see Figure 2). Since the limiting shifts (under conditions of excess LSR) also decrease as R becomes more bulky (Figure 2), this implies that there is a change in the geometry of the complex-LSR adduct with change of R. Undoubtedly such ; I variation will involve a lengthening of the complex-LSR distance as R becomes more bulky. Although a change of alkyl substituent will also result in concomitant electronic changes which may also influence Lidduct formation, it seems clear that the considerable differences observed between, for example, the isobutyl and trrt-butyl derivatives, occur mainly because of the different steric properties of these groups. While this is very likely the case for the series of alkyl derivatives, both steric and electronic effects could well be significant in the phenylsubstituted complex. To investigate the influence of electronic factors, the complex with R = CF3 was prepared since this relatively small group should withdraw electron density from the conjugated chelate ring and reduce the capacity of the oxygen donors to coordinate simultaneously to the LSR. Accordingly, no evidence for adduct formation was observed on addition of Eu(fod)3 or Pr(fod)3 to this complex in deuteriochloroform. T h e disulfur analog9 of (I, R = CH3) is also unaffected by either of the above shift reagents and this very likely is the result of electronic effects; the sulfur atoms being softer than oxygen will tend to back-donate electron density into the a-system of the chelate ring rather than to the respective lanthanide ions which, in any case, are known to prefer hard donor atoms.I0 A range of studies involving the use of LSR's (including chiral reagents) for conformational and other studies in related complexes will be reported soon.
Synthesis and Reactivity of a-Cyanoenamines. A Novel General Method for Preparing a-Diketones from Amides Sir: W e wish to report the utility of a-cyanoenamines as a hitherto unrecognized class of readily available and reactive synthetic intermediates which allow, inter alia, for a versatile and efficient method of synthesis of a-diketones. T h e choice of a starting material for the synthesis of acyanoenamines has been dictated by our recent studies' on a-chloroenamines, a class of reagents readily available from carboxamides. Indeed a-chloroenamines which already embody most of the structural features of a-cyanoenamines react as keteniminium chlorides and are therefore capable of undergoing nucleophilic substitution with cyanide ion. Thus 1 chloro-N,N-2-trimethylpropenylamine ( l a ) readily prepared' from N,N-dimethylisobutyramide (2a) was treated for 40 hr with 1.1 equiv of dry potassium cyanide in refluxing acetonitrile to give 1-cyano-N,N-2-trimethylpropenylamine (3a) (80% yield, bp 167O, UC=N 2205 cm-I). The substitution reaction worked equally well for the synthesis of 3b from l b (80% yield, bp 65' (0.3 Torr), U C e h 2200 cm-I). However, replacement of chloride by cyanide in the less electrophilic 1,2-dichloro-N,N-dimethylpropenylarnine (IC) required the presence of a Lewis acid catalyst. Zinc cyanide in refluxing chloroform worked satisfactorily and gave 3c in 60% yield (purified by GLPC, UC-N 2200 cm-I). This procedure (Scheme I, path a ) could not be applied to the synthesis of a-cyanoenamines derived from monosubstituted acetamides 2d,e since the a-chloroenamines ld,e are more difficult to prepare and easily undergo thermal or base-catalyzed dehydrochlorination. However, when the diScheme I
IKI/COCl*
Acknowledgments. W e wish to thank the Australian Research Grants Committee for support and M r v. Pickles (University of New South Wales) for assistance. References and Notes
R R'C path b
(1) C. C. Hinkley, J. Am. Chem. SOC.,91, 5160(1969). (2) See for example J. Paul, K. Schlogl, and W. Silhan, Monatsh. Chem., 103, 243 (1974); B. F. G. Johnson, J., Lewis, P. McArdle, and J. R. Norton, J. Chem. SOC..Dalton Trans., 1253 (1974). and references therein. (3) P.J. McCarthy and A. E. Martell, Inorg. Chem.; 8, 781 (1987). (4) R. G. Charles and R. C. Ohlmann. J. Inorg. Nucl. Chem., 27, 119 (1965);
Journal of t h e American Chemical Society
= c,
J,
97.8 ,J April 16, 1975
3
= R'=CH3 b. R = C2H5 ;R ' = Q H s R z C H 3 ;R'zCl c 2. R
d . R i C H 3 ; R'; H e. R = % H s j R ' = H
,N(CH3)2 C N
A pilh a
KCN/CHaCN Zn(CNI?/CHCIj.A Or
